A few methods have been proposed to deal with the sign
problem. For problems in the continuous space, there are
fixed-node approximation,[1]}, [2]} released node
methods,[3]} exact cancelation methods with Green's
function sampling,[4]} multilevel blocking
algorithm,[5]}, [6]} a hybrid path integral and basis set
method,[7]} various pseudopotential
approximations,[8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}
the general method for replacing integration over pure states by
integration over idempotent density matrices,[17]} and
method by introducing several images of the system,[18]}
global stationary phase approach.[19]} There are also a
number of methods for lattice
models.[20]}, [21]}, [22]} So far many efforts have
been devoted to tackle this problem, however, it remains being the
key bottleneck in using PIMC for many-fermion systems.
